The tick-borne Borreliae are the causative agents of relapsing fever and Lyme disease worldwide. Antibodies are the primary weapon of the mammalian immune system against both Borrelia infections. Some antibody responses have 'traditional' functions while others are more versatile and have novel functions and modes of action. A novel type of antibody to the Borrelia functions independently of the complement system in its bactericidal action, which is a unique and novel function for antibodies. The evidence for these bactericidal, complement independent antibodies is derived from both in vivo and in vitro studies. The main goal for this competitive renewal is to define the mechanism of action of the bactericidal, complement independent antibodies to Borrelia and to identify the cells that produce these antibodies. Both B. burgdorferi and relapsing fever Borreliae will be used for the experimental procedures. There is experimental evidence of colocalization of the outer surface and variable lipoproteins (Osp and Vmp) with cholesterol glycolipid domains in the outer membrane of the Borreliae. Depletion of the cholesterol leads to resistance to the bactericidal action of the antibodies. Therefore, the contribution of the Borrelia cholesterol glycolipids to the bactericidal action of the antibodies will be studied, particularly in the context of the outer membrane architecture of the antigenic determinants within the cholesterol-glycolipid domains. Imaging, structural, biophysical, and biochemical approaches will be used to determine the extent and nature of the ordered cholesterol glycolipid domains (lipid rafts) in the experiments planned for Specific Aim I. The existence of lipid rafts in a prokaryote such as Borrelia constitutes a paradigm shift in microbiology. The biogenesis of the bactericidal antibodies is the main goal of Specific Aim II. The cells that generate bactericidal antibodies can be distributed among the entire B cell repertoire. T cell independence of the bactericidal antibodies suggests the involvement of marginal zone B cells, but some of these antibodies appear to have undergone affinity maturation leading to the possibility of their being generated by other types of B cells, as well. At a time when the multiple functions of antibodies are being increasingly recognized and passive immunization is being revived as therapy for infectious and other diseases, the understanding of the versatile nature of the antibody response to Borrelia will be a significant contribution to host defense.